1. Field of the Invention
The present invention relates to an automatic focusing device for a camera having a zoom lens.
2. Description of Related Art
Recently developed cameras, particularly so-called compact cameras, are generally equipped with a zoom lens and an automatic focusing device as standard constituent elements.
A typical conventional automatic focusing device comprises a distance measuring device for measuring the distance to an object and a focusing lens driving device for moving a focusing lens to a position where the lens is focused on the objects at the distance measured in the distance measuring device. The distance measuring device is, generally, activated to begin a distance measuring operation when the release button is depressed halfway. The distance measuring device repeatedly projects light and performs calculations for distance measurement a predetermined number of times (about 5 to 8 times) to obtain an average value of the object distance values. The focusing lens driving device moves the focusing lens to a position where the lens is focused on the object at the average distance value thus obtained.
Thus, the conventional cameras perform the distance measuring operation repeatedly and therefore take a long time to complete focusing.
In the meantime, the photographing lens has such a depth of field that, even if the lens is not exactly focused, it can be virtually regarded as focused.
As is generally known, depth of field has the nature that the shorter the focal length, the greater the depth of field. The depth of field also has the nature that the larger the aperture value (F-number), the greater the depth of field. In other words, in the same zoom lens the depth of field at the wide-angle end (the shortest focal length) is greater than that at the telephoto end (the longest focal length).
Accordingly, it is necessary in order to measure a precise object distance, execute the of distance measurement at the telephoto and more times than at the wide-angle end.
However, in conventional automatic focusing device, the number of times of execution of distance measurement is fixed irrespective of the depth of field. Accordingly, if the number of times of execution of distance measurement is set in conformity to the telephoto end, at the wide-angle end distance measurement is needlessly performed and the time required for focusing therefore becomes needlessly longer. Moreover, more current is consumed than is actually needed. On the other hand, if the number of distance measurements is set in conformity to the wide-angle end, the number of measurements is insufficient to attain precise focusing at the telephoto end.